In EUV lithography, highly integrated structures are generated by electromagnetic radiation. The wavelength of the radiation used for this purpose is in a range of approximately 13 nm, i.e., in the range of extreme ultraviolet light (EUV). EUV mirrors are used in EUV projection systems and comprise at least one substrate and one reflective multilayer coating that is applied onto the substrate. For this purpose, the substrate regularly comprises a zero-expansion material and location-independent zero crossing temperatures of thermal expansion in the substrate. Upon exposure to an extreme ultraviolet radiation in an EUV projection system, the EUV mirror comprises location-dependent operating temperatures.
The mirror substrates usually consists of glass that has a high silicic acid content and is doped with titanium dioxide. The actual mirror is obtained by mechanical processing and coating of the substrate with a multilayer coating. The maximal (theoretical) reflectivity of an EUV mirror element in this context is approximately 70%, such that approximately 30% of the radiation energy is absorbed and converted into heat in the coating or in the mirror substrate layers close to the surface. The radiation penetrating into the mirror substrate elicits thermal changes of the substrate volume, which can be associated with surface deformations. Even minute volume changes with a magnitude of 1 nm can lead to noticeable deterioration and distortion of the imaging quality. The surface region around the impact surface of the radiation is particularly strongly strained in this context.
DE 10 2010 009 589 B4 describes a method for producing a blank made of titanium-doped glass with a high silicic acid content for a mirror substrate for use in EUV lithography. The mirror substrate comprises a surface region that is to be mirror coated, whereby the glass for said surface region is produced by flame hydrolysis and a pre-defined hydrogen content is established.
US 2012/0264584 A1 discloses a substrate for an EUV lithography projection system comprising two different surfaces whose zero crossing temperatures differ by at least 5° C.
DE 10 2011 085 358 B3 relates to an optical arrangement for EUV lithography and a method for the configuration thereof. In this context, the optical arrangement, for example a projection objective lens, comprises two mirror substrates having different thermal expansion coefficients that are each arranged appropriately such that the distortion caused by deformation of the mirrors is compensated.